[llvm-dev] [RFC] Add IR level interprocedural outliner for code size.

[River Riddle via llvm-dev]

Hi Chris,

It's okay to put this on the spot because posting the patches was meant to
help further the discussion that kind of stalled previously.

On Fri, Jul 28, 2017 at 9:58 PM, Chris Bieneman <beanz at apple.com> wrote:

> Apologies for delayed joining of this discussion, but I had a few notes
> from this thread that I really wanted to chime in about.
>
> River,
>
> I don't mean to put you on the spot, but I do want to start on a semantic
> issue. In several places in the thread you used the words "we" and "our" to
> imply that you're not alone in writing this (which is totally fine), but
> your initial thread presented this as entirely your own work. So, when you
> said things like "we feel there's an advantage to being at the IR level",
> can you please clarify who is "we"?
>

 In regards to the words "we" and "our", I am referring to myself. My
writing style tends to shift between the usage of those words. I'll avoid
any kind of confusion in the future.


> Given that there are a number of disagreements and opinions floating
> around I think it benefits us all to speak clearly about who is taking what
> stances.
>
> One particular disagreement that I think very much needs to be revisited
> in this thread was Jessica's proposal of a pipeline of:
>
>    1. IR outline
>    2. Inline
>    3. MIR outline
>
> In your response to that proposal you dismissed it out of hand with
> "feelings" but not data. Given that the proposal came from Jessica (a
> community member with significant relevant experience in outlining), and it
> was also recognized as interesting by Eric Christopher (a long-time member
> of the community with wide reaching expertise), I think dismissing it may
> have been a little premature.
>

I dismissed the idea of an outliner at the machine level being able to
catch bad inlining decisions. Given the loss of information between the two
I felt it was a little optimistic to rely on a very late pass being able to
reverse those decisions, especially coupled with the fact that the current
machine outliner requires exact equivalence. I don't disagree with the
proposal of an example : outline, inline, outline: pipeline, but the idea
of being able to catch inlining decisions given the circumstances seemed
optimistic to me. From there I went ahead and implemented a generic
interface for outlining that can be shared between IR/Machine level so that
such a pipeline could be more feasible.


>
> I also want to visit a few procedural notes.
>
> Mehdi commented on the thread that it wouldn't be fair to ask for a
> comparative study because the MIR outliner didn't have one. While I don't
> think anyone is asking for a comparative study, I want to point out that I
> think it is completely fair. If a new contributor approached the community
> with a new SROA pass and wanted to land it in-tree it would be appropriate
> to ask for a comparative analysis against the existing pass. How is this
> different?
>

The real question comes from what exactly you want to define as a
"comparative analysis". When posting the patch I included additional
performance data( found here goo.gl/5k6wsP) that includes benchmarking and
comparisons between the outliner that I am proposing and the machine
outliner on a wide variety of benchmarks. The proposed outliner performs
quite favorable in comparison. As for feature comparison, the proposed
outliner has many features currently missing from the machine outliner:
 - parameterization
 - outputs
 - relaxed equivalence(machine outliner requires exact)
 - usage of profile data
 - support for opt remarks

 The machine outliner currently only supports X86 and AArch64, the IR
outliner can/should support all targets immediately without the requirement
of ABI restrictions(mno-red-zone is required for the machine outliner).
 At the IR level we have much more opportunity to find congruent
instructions than at the machine level given the possible variation at that
level: RA, instruction selection, instruction scheduling, etc.


I am more than willing to do a comparative analysis but I'm not quite sure
what the expectation for one is.


> Adding a new IR outliner is a different situation from when the MIR one
> was added. When the MIR outliner was introduced there was no in-tree
> analog. When someone comes to the community with something that has no
> existing in-tree analog it isn't fair to necessarily ask them to implement
> it multiple different ways to prove their solution is the best. However, as
> a community, we do still exercise the right to reject contributions we
> disagree with, and we frequently request changes to the implementation (as
> is shown every time someone tries to add SPIR-V support).
>

I perfectly agree :)


> In the LLVM community we have a long history of approaching large
> contributions (especially ones from new contributors) with scrutiny and
> discussion. It would be a disservice to the project to forget that.
>
> River, as a last note. I see that you've started uploading patches to
> Phabricator, and I know you're relatively new to the community. When
> uploading patches it helps to include appropriate reviewers so that the
> right people see the patches as they come in. To that end can you please
> include Jessica as a reviewer? Given her relevant domain experience I think
> her feedback on the patches will be very valuable.
>

I accidentally posted without any reviewers at first, I've been going back
through and adding people I missed.


> Thank you,
> -Chris
>
I appreciate the feedback and welcome all critical discussion about the
right way to move forward.
Thanks,
 River Riddle


>
> On Jul 26, 2017, at 1:52 PM, River Riddle via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Hey Sanjoy,
>
> On Wed, Jul 26, 2017 at 1:41 PM, Sanjoy Das via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hi,
>>
>> On Wed, Jul 26, 2017 at 12:54 PM, Sean Silva <chisophugis at gmail.com>
>> wrote:
>> > The way I interpret Quentin's statement is something like:
>> >
>> > - Inlining turns an interprocedural problem into an intraprocedural
>> problem
>> > - Outlining turns an intraprocedural problem into an interprocedural
>> problem
>> >
>> > Insofar as our intraprocedural analyses and transformations are strictly
>> > more powerful than interprocedural, then there is a precise sense in
>> which
>> > inlining exposes optimization opportunities while outlining does not.
>>
>> While I think our intra-proc optimizations are *generally* more
>> powerful, I don't think they are *always* more powerful.  For
>> instance, LICM (today) won't hoist full regions but it can hoist
>> single function calls.  If we can extract out a region into a
>> readnone+nounwind function call then LICM will hoist it to the
>> preheader if the safety checks pass.
>>
>> > Actually, for his internship last summer River wrote a profile-guided
>> > outliner / partial inliner (it didn't try to do deduplication; so it was
>> > more like PartialInliner.cpp). IIRC he found that LLVM's interprocedural
>> > analyses were so bad that there were pretty adverse effects from many
>> of the
>> > outlining decisions. E.g. if you outline from the left side of a
>> diamond,
>> > that side basically becomes a black box to most LLVM analyses and forces
>> > downstream dataflow meet points to give an overly conservative result,
>> even
>> > though our standard intraprocedural analyses would have happily dug
>> through
>> > the left side of the diamond if the code had not been outlined.
>> >
>> > Also, River's patch (the one in this thread) does parameterized
>> outlining.
>> > For example, two sequences containing stores can be outlined even if the
>> > corresponding stores have different pointers. The pointer to be loaded
>> from
>> > is passed as a parameter to the outlined function. In that sense, the
>> > outlined function's behavior becomes a conservative approximation of
>> both
>> > which in principle loses precision.
>>
>> Can we outline only once we've already done all of these optimizations
>> that outlining would block?
>>
>
>   The outliner is able to run at any point in the interprocedural
> pipeline. There are currently two locations: Early outlining(pre inliner)
> and late outlining(practically the last pass to run). It is configured to
> run either Early+Late, or just Late.
>
>
>> > I like your EarlyCSE example and it is interesting that combined with
>> > functionattrs it can make a "cheap" pass get a transformation that an
>> > "expensive" pass would otherwise be needed. Are there any cases where we
>> > only have the "cheap" pass and thus the outlining would be essential
>> for our
>> > optimization pipeline to get the optimization right?
>> >
>> > The case that comes to mind for me is cases where we have some cutoff of
>> > search depth. Reducing a sequence to a single call (+ functionattr
>> > inference) can essentially summarize the sequence and effectively
>> increase
>> > search depth, which might give more results. That seems like a bit of a
>> weak
>> > example though.
>>
>> I don't know if River's patch outlines entire control flow regions at
>> a time, but if it does then we could use cheap basic block scanning
>> analyses for things that would normally require CFG-level analysis.
>>
>
>   The current patch currently just supports outlining from within a single
> block. Although, I had a working prototype for Region based outlining, I
> kept it from this patch for simplicity. So its entirely possible to add
> that kind of functionality because I've already tried.
> Thanks,
>   River Riddle
>
>
>>
>> -- Sanjoy
>>
>> >
>> > -- Sean Silva
>> >
>> > On Wed, Jul 26, 2017 at 12:07 PM, Sanjoy Das via llvm-dev
>> > <llvm-dev at lists.llvm.org> wrote:
>> >>
>> >> Hi,
>> >>
>> >> On Wed, Jul 26, 2017 at 10:10 AM, Quentin Colombet via llvm-dev
>> >> <llvm-dev at lists.llvm.org> wrote:
>> >> > No, I mean in terms of enabling other optimizations in the pipeline
>> like
>> >> > vectorizer. Outliner does not expose any of that.
>> >>
>> >> I have not made a lot of effort to understand the full discussion here
>> (so
>> >> what
>> >> I say below may be off-base), but I think there are some cases where
>> >> outlining
>> >> (especially working with function-attrs) can make optimization easier.
>> >>
>> >> It can help transforms that duplicate code (like loop unrolling and
>> >> inlining) be
>> >> more profitable -- I'm thinking of cases where unrolling/inlining would
>> >> have to
>> >> duplicate a lot of code, but after outlining would require duplicating
>> >> only a
>> >> few call instructions.
>> >>
>> >>
>> >> It can help EarlyCSE do things that require GVN today:
>> >>
>> >> void foo() {
>> >>   ... complex computation that computes func()
>> >>   ... complex computation that computes func()
>> >> }
>> >>
>> >> outlining=>
>> >>
>> >> int func() { ... }
>> >>
>> >> void foo() {
>> >>   int x = func();
>> >>   int y = func();
>> >> }
>> >>
>> >> functionattrs=>
>> >>
>> >> int func() readonly { ... }
>> >>
>> >> void foo(int a, int b) {
>> >>   int x = func();
>> >>   int y = func();
>> >> }
>> >>
>> >> earlycse=>
>> >>
>> >> int func(int t) readnone { ... }
>> >>
>> >> void foo(int a, int b) {
>> >>   int x = func(a);
>> >>   int y = x;
>> >> }
>> >>
>> >> GVN will catch this, but EarlyCSE is (at least supposed to be!)
>> cheaper.
>> >>
>> >>
>> >> Once we have an analysis that can prove that certain functions can't
>> trap,
>> >> outlining can allow LICM etc. to speculate entire outlined regions out
>> of
>> >> loops.
>> >>
>> >>
>> >> Generally, I think outlining exposes information that certain regions
>> of
>> >> the
>> >> program are doing identical things.  We should expect to get some
>> mileage
>> >> out of
>> >> this information.
>> >>
>> >> -- Sanjoy
>> >> _______________________________________________
>> >> LLVM Developers mailing list
>> >> llvm-dev at lists.llvm.org
>> >> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> >
>> >
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>
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